Semiconductor light-emitting device and apparatus for driving the same

ABSTRACT

A semiconductor light-emitting device has first and second semiconductor layers each of a first conductivity type, a third semiconductor layer of a second conductivity type provided between the first and second semiconductor layers, and an active layer provided between the second and third semiconductor layers to emit light with charge injected therein from the second and third semiconductor layers. A graded composition layer is provided between the active layer and the third semiconductor layer to have a varying composition which is nearly equal to the composition of the active layer at the interface with the active layer and to the composition of the third semiconductor layer at the interface with the third semiconductor layer.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a semiconductor light-emittingdevice in a triode configuration such as a light-emitting diode deviceor a semiconductor laser device and to an apparatus for driving thesame.

[0002] Light-emitting diode devices have been used widely as low-costand high-reliability light-emitting devices in remote control equipmentand optical fiber communication.

[0003] However, conventional light-emitting diode devices have theproblems of low response speed and low upper-limit modulation frequencyin performing high-speed communication, i.e., high-speed modulation.

[0004] Factors that limit the operating speed of a semiconductorlight-emitting device represented by a light-emitting diode deviceinclude the speed at which carriers injected in the active layer arerecombined. The carriers injected in the active region of thelight-emitting device do not disappear immediately after currentinjection is halted but disappear gradually in accordance with a timeconstant determined by the recombination speed.

[0005] Since the light-emitting state continues while the carriersremain in the active region, the carriers remaining in the active regionprevent high-speed response of the light-emitting device duringmodulation. Since the light-emitting diode device utilizes spontaneouslight emission and the amount of light emitted therefrom is nearlyproportional to the quantity of carriers in the active region, theremaining carriers exert particularly great influence on the responsespeed of the light-emitting diode device. In a light-emitting diodedevice composed of a Group III-V compound semiconductor containing,e.g., aluminium gallium arsenide (AlGaAs) as a main component, the timeconstant determined by the carrier recombination speed is normallyseveral nanoseconds (ns) so that it is difficult to perform high-speedmodulation at a modulation frequency exceeding 1 GHz.

[0006] As prior art technology for eliminating the limit placed by thecarrier recombination speed on the modulation speed, a light-emittingdevice using a triode configuration similar to that of a transistordevice is disclosed in Japanese Unexamined Patent Publication No. SHO60-167390.

[0007]FIG. 17 shows a cross-sectional structure of the triodelight-emitting device disclosed in the publication.

[0008] As shown in FIG. 17, the semiconductor light-emitting devicedisclosed in the publication comprises a p-type collector layer 902, ann-type base layer 903, and a p-type emitter layer 905 formedsuccessively on a p-type semiconductor substrate 901, similarly to abipolar transistor.

[0009] An active layer 904 is provided between the base layer 903 andthe emitter layer 905. The active layer 904 is surrounded by an n-typeburied layer 907 formed in the peripheral region thereof.

[0010] An emitter electrode 909 is formed on the emitter layer 905 witha p-type contact layer 906 interposed therebetween. A base electrode 910is formed on the buried layer 907 with an n-type contact layer 908interposed therebetween so as to surround the emitter electrode 909. Acollector electrode 911 is formed on the surface of the semiconductorsubstrate 901 opposite to the collector layer 902.

[0011] A description will be given herein below to the operation of theconventional semiconductor light-emitting device.

[0012]FIG. 18 shows the structure of electron energy bands in theconventional semiconductor light-emitting device during a light-emittingperiod, in which the vertical axis represents the energy of electronsand E_(c), E_(v), and E_(F) generally represent energy at the lower endof the conduction band, energy at the upper end of the valence band, andthe energy of electrons or holes on a quasi-Fermi level, respectively.The reference numerals associated with the energy levels correspond tothe semiconductor layers shown in FIG. 17.

[0013] As an example of a driving voltage applied during thelight-emitting period, a voltage in a forward direction (forward biasvoltage) is applied between the base layer 904 and the emitter layer 905such that the base layer 904 and the collector layer 902 are set at anequal potential of 0 V.

[0014] Since the forward bias voltage is applied between the base layer903 and the emitter layer 905, electrons injected from the base layer903 and holes injected from the emitter layer 905 are accumulated in theactive layer 904 and recombined to emit light. Although a depletionlayer is formed between the p-type collector layer 902 and the n-typebase layer 903 due to the pn junction, at least a part of the base layer903 is not depleted so that the electrons are supplied from theundepleted portion to the active layer 904. The base layer 903 functionsas a barrier for confining the holes to the active layer.

[0015] During a light-extinct period, a voltage in a reverse direction(reverse bias voltage) is applied between the base layer 903 and thecollector layer 902. This depletes substantially the entire region ofthe base layer 904, as shown in the energy-band diagram of FIG. 19, sothat the holes confined to the active layer 904 are extracted to thecollector layer 902. If the holes can be extracted from the active layer904 with sufficiently high efficiency, the concentration of the holes inthe active layer 904 is reduced so that the quantity of carriersrecombined for light emission is reduced and light emission issuppressed. Since the hole extracting operation is not dependent on thespeed of carrier recombination for light emission, light emission can behalted promptly so that high-speed modulation is allowed.

[0016] As a result of conducting various studies on the conventionalsemiconductor light-emitting device in the triode configuration, thepresent inventors have found the problem that, if low-voltage driving isperformed during a light-extinct operation, some of the holes remain inthe active layer 904 and emitted light remains even during theextinction period. Briefly, it is difficult to achieve a high extinctionratio, which is the ratio between the amount of light during thelight-emitting period and the amount of light during the extinctionperiod.

[0017]FIG. 20 shows in enlarged relation a band structure at the upperend of the valence band in the active layer 904 and its vicinity in theconventional semiconductor light-emitting device during the extinctionperiod. As shown in FIG. 20, an interface barrier (spike) 920 occursbetween the active layer 904 and the base layer 903 during theextinction period due to the offsetting of the valence band caused bythe heterojunction therebetween. Even if the absolute value of thepotential of the reverse bias voltage applied to the collector layer 902is increased, the height of the interface barrier 902 (the magnitude ofenergy) does not change, which forms an obstacle to the extraction ofthe holes to the collector layer 902. Although some of the holes movetoward the collector by surpassing the interface barrier 902 with thereverse bias voltage, holes with energy lower than the height of theinterface barrier 902 remain at the interface between the active layer904 and the base layer 903. If a higher reverse bias voltage is applied,some of the holes with lower energy are transported by a tunnel currentto the collector layer 902 but the reverse bias voltage with the higherabsolute value also increases the amount of heat generated from thedevice as well as power consumption.

[0018] At this time, the holes are supplied from the emitter layer 905to the active layer 904 so that, if the concentration of the holes isincreased at the interface between the active layer 904 and the baselayer 903, the quantity of holes accumulated in the entire active layer904 is increased. In the conventional semiconductor light-emittingdevice, therefore, it is difficult to sufficiently reduce the quantityof holes in the active layer 904 with a low reverse bias voltage and aconsiderable amount of light is emitted from the active layer 904 evenduring the extinction period.

[0019] Thus, it is difficult to achieve a higher extinction ratio in theconventional semiconductor light-emitting device in the triodeconfiguration during the low-voltage driving.

SUMMARY OF THE INVENTION

[0020] It is therefore an object of the present invention to allowhigh-speed operation with a low voltage and provide a practicalextinction ratio by solving the conventional problems.

[0021] To attain the foregoing object, a first semiconductorlight-emitting device according to the present invention comprises:first and second semiconductor layers each of a first conductivity type;a third semiconductor layer of a second conductivity type providedbetween the first and second semiconductor layers; an active layerprovided between the second and third semiconductor layers, the activelayer emitting light with charge injected therein from the second andthird semiconductor layers; and a graded composition layer providedbetween the active layer and the third semiconductor layer to have avarying composition which is nearly equal to a composition of the activelayer at an interface with the active layer and to a composition of thethird semiconductor layer at an interface with the third semiconductorlayer.

[0022] If the third semiconductor layer of the known semiconductorlight-emitting device is a base layer, the active layer and the baselayer are composed of heterojunctions, as described above. Accordingly,a band offset causes an interface barrier when a reverse bias voltage isapplied during an extinction period. However, the first semiconductorlight-emitting device of the present invention has the gradedcomposition layer between the active layer and the third semiconductorlayer, which eliminates the band offset and therefore prevents theoccurrence of the interface barrier. As a result, even a low reversebias voltage achieves a sufficient reduction in the quantity of carriersremaining in the active layer so that a higher extinction ratio isachieved by low-voltage driving.

[0023] A second semiconductor light-emitting device according to thepresent invention comprises: first and second semiconductor layers eachof a first conductivity type; a third semiconductor layer of a secondconductivity type provided between the first and second semiconductorlayers, the third semiconductor layer having a forbidden band as anelectron energy band which is smaller in width than a forbidden band ineach of the first and second semiconductor layers; and a gradedcomposition layer provided between the first and third semiconductorlayers to have a varying composition which is nearly equal to acomposition of the first semiconductor layer at an interface with thefirst semiconductor layer and to a composition of the thirdsemiconductor layer at an interface with the third semiconductor layer,the third semiconductor layer emitting light with charge injectedtherein from the second and third semiconductor layers.

[0024] If the third semiconductor layer of the second semiconductorlight-emitting device is the base layer, the base layer functions as asubstantial active layer since the forbidden band width in the baselayer is smaller than the forbidden band width in each of the first andsecond semiconductor layers. Thus, even in the semiconductorlight-emitting device which does not have an independent active layer,the graded composition layer provided between the first semiconductorlayer (collector layer) and the third semiconductor layer (base layer)eliminates the band offset and therefore prevents the occurrence of theinterface barrier. As a result, even a low reverse bias voltage achievesa sufficient reduction in the quantity of carriers remaining in thethird semiconductor layer so that a higher extinction ratio is achievedby low-voltage driving.

[0025] A third semiconductor light-emitting device according to thepresent invention comprises: first and second semiconductor layers eachof a p-type conductivity; and a third semiconductor layer of an n-typeconductivity provided between the first and second semiconductor layers,the third semiconductor layer having a forbidden band as an electronenergy band which is smaller in width than a forbidden band in each ofthe first and second semiconductor layers, the third semiconductor layeremitting light with charge injected therein from the second and thirdsemiconductor layers, an energy value at an upper end of a valence bandas an electron energy band being lower in the first semiconductor layerthan in the second semiconductor layer.

[0026] In the third semiconductor light-emitting device, the thirdsemiconductor layer functions as a substantial active layer, similarlyto the second semiconductor light-emitting device of the presentinvention. If the first semiconductor layer is a collector layer and thesecond semiconductor layer is an emitter layer, an energy value at theupper end of the valence band is lower in the collector layer as thefirst semiconductor layer than in the emitter layer as the secondsemiconductor layer. This suppresses current injection from thecollector layer without interrupting current injection from the emitterduring a light-emitting period. This also suppresses a leakage currentfrom the emitter layer to the collector layer and achieves a higherextinction ratio.

[0027] A fourth semiconductor light-emitting device according to thepresent invention comprises: first and second semiconductor layers eachof an n-type conductivity; and a third semiconductor layer of a p-typeconductivity provided between the first and second semiconductor layers,the third semiconductor layer having a forbidden band as an electronenergy band which is smaller in width than a forbidden band in each ofthe first and second semiconductor layers, the third semiconductor layeremitting light with charge injected therein from the second and thirdsemiconductor layers, an energy value at a lower end of a conductionband as an electron energy band being higher in the first semiconductorlayer than in the second semiconductor layer.

[0028] In the fourth semiconductor light-emitting device, the thirdsemiconductor layer functions as a substantial active layer, similarlyto the second semiconductor light-emitting device of the presentinvention. If the first semiconductor layer is a collector layer and thesecond semiconductor layer is an emitter layer, an energy value at thelower end of the conduction band as an electron energy band is higher inthe collector layer as the first semiconductor layer than in the emitterlayer as the second semiconductor layer. This suppresses currentinjection from the collector layer without interrupting currentinjection from the emitter during the light-emitting period. This alsosuppresses a leakage current from the emitter layer to the collectorlayer and achieves a higher extinction ratio.

[0029] In each of the second to fourth semiconductor light-emittingdevices, an impurity concentration in the second semiconductor layer ispreferably higher at least in a region thereof opposed to the firstsemiconductor layer than in the first semiconductor layer. If the firstsemiconductor layer is a collector layer and the second semiconductorlayer is an emitter layer, the second semiconductor layer is higher inimpurity concentration than in the first semiconductor layer so that theefficiency of carrier injection from the second semiconductor layer(emitter layer) is improved.

[0030] A fifth semiconductor light-emitting device according to thepresent invention comprises: first and second semiconductor layers eachof a first conductivity type; a third semiconductor layer of a secondconductivity type provided between the first and second semiconductorlayers, the third semiconductor layer having a forbidden band as anelectron energy band which is smaller in width than a forbidden band ineach of the first and second semiconductor layers; and a lightly dopedsemiconductor layer provided between the first and third semiconductorlayers, the lightly doped semiconductor layer having an impurityconcentration which is lower than an impurity concentration in each ofthe first and third semiconductor layers, the third semiconductor layeremitting light with charge injected therein from the second and thirdsemiconductor layers.

[0031] In the fifth semiconductor light-emitting device, the thirdsemiconductor layer functions as a substantial active layer, similarlyto the second semiconductor light-emitting device of the presentinvention. If the first semiconductor layer is a collector layer, thepotential gradient in the interface barrier between the thirdsemiconductor layer (base layer) and the first semiconductor layer(collector layer) becomes sharp during the extinction period due to thelightly doped semiconductor layer provided between the first and thirdsemiconductor layers, which prevents carriers from remaining in theinterface barrier portion. As a result, even a low reverse bias voltageachieves a sufficient reduction in the quantity of carriers remaining inthe third semiconductor layer so that a higher extinction ratio isachieved by low-voltage driving.

[0032] In the fifth semiconductor light-emitting device, the lightlydoped semiconductor layer is preferably an undoped layer undoped with animpurity.

[0033] In the fifth semiconductor light-emitting device, the lightlydoped semiconductor layer preferably has the second conductivity type.In the arrangement, the lightly doped semiconductor layer providedbetween the first semiconductor layer (collector layer) and the thirdsemiconductor layer (base layer) forms a pn junction between itself andthe first semiconductor layer. During the light-emitting period,therefore, a barrier against carriers injected from the firstsemiconductor layer (collector layer) to the third semiconductor layer(base layer) occurs during the light-emitting period even with theapplication of a forward bias voltage between the collector and thebase. The barrier prevents carrier injection in a reverse direction fromthe first semiconductor layer (collector layer) even if the firstsemiconductor layer (collector layer) and the second semiconductor layer(emitter layer) are set at equal values.

[0034] An apparatus for driving a semiconductor light-emitting deviceaccording to the present invention assumes an apparatus for driving asemiconductor light-emitting device comprising first and secondsemiconductor layers each of a first conductivity type and a thirdsemiconductor layer of a second conductivity type provided between thefirst and second semiconductor layers, the apparatus comprising:constant-current control means; light-emission control means forcontrolling a state of light emitted from the semiconductorlight-emitting device; and specified-potential applying means forapplying a specified potential to the third semiconductor layer of thesemiconductor light-emitting device, the constant-current control meanssupplying a specified driving current to the second semiconductor layerof the semiconductor light-emitting device, the light-emission controlmeans adjusting an amount of light emitted from the semiconductorlight-emitting device by applying different voltages to the firstsemiconductor layer or by bringing the first semiconductor layer intodifferent states of impedance.

[0035] The apparatus for driving a semiconductor light-emitting deviceaccording to the present invention ensures the light-emitting andlight-extinct operations of a semiconductor light-emitting device in atriode configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036]FIG. 1 is a structural cross-sectional view of a pnp-typesemiconductor light-emitting device in a triode configuration accordingto a first embodiment of the present invention;

[0037]FIG. 2 is a band diagram showing the energy of electrons at an endof the valence band in an active layer and its vicinity in thesemiconductor light-emitting device according to the first embodimentduring an extinction period;

[0038]FIG. 3 is a structural cross-sectional view of an npn-typesemiconductor light-emitting device in a triode configuration accordingto a second embodiment of the present invention;

[0039]FIG. 4 is a structural cross-sectional view of a pnp-typesemiconductor light-emitting device in a triode configuration accordingto a third embodiment of the present invention;

[0040]FIG. 5 is a band diagram of electron energy bands in thesemiconductor light-emitting device according to the third embodimentduring a light-emitting period;

[0041]FIG. 6 is a band diagram of the electron energy bands in thesemiconductor light-emitting device according to the third embodimentduring the extinction period;

[0042]FIG. 7 is a structural cross-sectional view of an npn-typesemiconductor light-emitting device in a triode configuration accordingto a fourth embodiment of the present invention;

[0043]FIG. 8 is a structural cross-sectional view of a pnp-typesemiconductor light-emitting device in a triode configuration accordingto a fifth embodiment of the present invention;

[0044]FIG. 9A is a band diagram of the electron energy bands in thesemiconductor light-emitting device according to the fifth embodimentduring the extinction period;

[0045]FIG. 9B is a band diagram of the electron energy bands in acomparative semiconductor light-emitting device without an undopedsemiconductor layer during the extinction period;

[0046]FIG. 10 is a structural cross-sectional view of a pnp-typesemiconductor light-emitting device in a triode configuration accordingto a sixth embodiment of the present invention;

[0047]FIG. 11 is a band diagram of the electron energy bands in thesemiconductor light-emitting device according to the sixth embodimentduring the light-emitting period;

[0048]FIG. 12 is a band diagram of the electron energy bands in thesemiconductor light-emitting device according to the sixth embodimentduring the extinction period;

[0049]FIG. 13 is a band diagram of the electron energy bands in thesemiconductor light-emitting device according to a variation of thesixth embodiment during the light-emitting period;

[0050]FIG. 14 is a band diagram of the electron energy bands in thesemiconductor light-emitting device according to the variation of thesixth embodiment during the extinction period;

[0051]FIG. 15 is a functional block diagram of an apparatus for drivinga semiconductor light-emitting device according to a seventh embodimentof the present invention;

[0052]FIG. 16 is a functional block diagram of an apparatus for drivinga semiconductor light-emitting device according to an eighth embodimentof the present invention;

[0053]FIG. 17 is a structural cross-sectional view of a conventionalsemiconductor light-emitting device in a triode configuration;

[0054]FIG. 18 is a band diagram of the electron energy bands in theconventional semiconductor light-emitting device during thelight-emitting period;

[0055]FIG. 19 is a band diagram of the electron energy bands in theconventional semiconductor light-emitting device during thelight-emitting period; and

[0056]FIG. 20 is a band diagram showing the electron energy bands at anend of the valence band in an active layer and its vicinity in theconventional semiconductor light-emitting device and its vicinity duringthe extinction period.

DETAILED DESCDRIPTION OF THE INVENTION

[0057] Embodiment 1

[0058] A first embodiment of the present invention will be describedwith reference to the drawings.

[0059] In each of the embodiments of the present invention, the sameterminology as used for a bipolar transistor will be used to refer tothe three semiconductor layers of a semiconductor light-emitting devicein a triode configuration. That is, a first semiconductor layer of afirst conductivity type is termed a collector layer, a secondsemiconductor layer of the first conductivity type is termed an emitterlayer, and a third semiconductor layer of a second conductivity type istermed a base layer.

[0060]FIG. 1 shows a cross-sectional structure of a semiconductorlight-emitting device in a triode configuration according to the firstembodiment, which is a pnp-type semiconductor light-emitting devicecomposed of a GaAs/GaInP-based compound semiconductor.

[0061] As shown in FIG. 1, the semiconductor light-emitting deviceaccording to the first embodiment has a collector layer 102 composed ofp-type gallium indium phosphide (GaInP), a base layer 103 composed ofn-type gallium indium phosphide (GaInP) with a thickness of about 300nm, a graded composition layer 104 with a thickness of about 50 nm, anactive layer 105 composed of GaAs with a thickness of about 100 nm, andan emitter layer 106 composed of p-type GaInP which are formedsuccessively on a substrate 101 composed of p-type gallium arsenide(GaAs).

[0062] The first embodiment features the graded composition layer 104provided between the base layer 103 and the active layer 105 to have acomposition which is nearly equal to the composition of the base layer103 at the interface with the base layer 103 and to the composition ofthe active layer 105 at the interface with the active layer 105. If thefilm thickness of the graded composition layer 104 is about 5 nm toabout 100 nm, the occurrence of an interface barrier can be suppressed.The composition of the graded composition layer 104 may be variedcontinuously or stepwise. Since light is emitted from the region of thegraded composition layer 104 closer to the active layer 105, it is alsopossible to regard the region as a part of the active layer 105.

[0063] A p-type collector electrode 108 is formed on the surface of thesubstrate 101 opposite to the collector layer 102.

[0064] The upper surface of the active layer 105 is exposed and ann-type base electrode 109 is formed on the exposed region in spacedapart relation to the side surface of the emitter layer 106. Thus, then-type base electrode 109 according to the first embodiment is providednot directly on the upper surface of the base layer 103 but with thegraded composition layer 104 and active layer 105 interposedtherebetween. Since the active layer 105 having the forbidden band as anelectron energy band which is smaller in width than the forbidden bandin the base layer 103 is used as a substantial contact layer for then-type base electrode 109, the contact resistance of the n-type baseelectrode 109 can be reduced.

[0065] To give higher priority to the efficiency of electron injection,the regions of the active layer 105 and the graded composition layer 104lying between the n-type base electrode 109 and the emitter layer 106are left unremoved. However, it is also possible to improve theefficiency with which holes injected from the emitter layer 106 areconfined by removing the regions.

[0066] A p-type contact layer 107 composed of p-type high-concentrationGaAs is formed on a part of the emitter layer 106. A p-type emitterelectrode 110 is formed on the p-type contact layer 107.

[0067] In the first embodiment, an impurity concentration in each of thegraded composition layer 104 and the active layer 105 is adjusted toabout 6×10¹⁶ cm⁻³ and an impurity concentration in each of the collectorlayer 102, the base layer 103, and the emitter layer 106 is adjusted toabout 1×10¹⁷ cm⁻³.

[0068] In the first embodiment, a mixed crystal composed of GaInP havinga composition which substantially lattice-matches with the substrate 101composed of GaAs is used for the collector layer 102, the base layer103, and the emitter layer 106. This reduces the resistivity of each ofthe collector layer 102, the base layer 103, and the emitter layer 106to a low value, while allowing a large band offset between the activelayer 105 composed of GaAs and each of the collector layer 102, the,base layer 103, and the emitter layer 106 each composed of GaInP.

[0069] Since the base layer 103 and the active layer 105 are provideddiscretely, the resistance of carriers injected from the n-type baseelectrode 109 in a direction parallel to the substrate surface isreduced, which suppresses delayed operation and increased powerconsumption each resulting from device resistance.

[0070] A description will be given herein below to the light-emittingoperation and light-extinct operation of the semiconductorlight-emitting device thus constituted.

[0071] During a light-emitting period, the semiconductor light-emittingdevice according to the first embodiment applies a forward bias voltagebetween the base layer 103 and the emitter layer 106 and adjusts thepotential between the base layer 103 and the collector layer 102 to 0 V,thereby confining carriers to the active layer 105. The confinedcarriers, i.e., electrons and holes are recombined in the active layer105, thereby causing light emission.

[0072] During an extinction period, a reverse bias voltage is appliedbetween the base layer 103 and the collector layer 102. FIG. 2 shows aband structure at an end of the valence band in the active layer 105 andits vicinity. In FIG. 2, reference numerals associated with the energylevels correspond to the semiconductor layers shown in FIG. 1.

[0073] As shown in FIG. 2, the graded composition layer 104 is providedbetween the active layer 105 and the base layer 103 to have a graduallyvarying composition which is nearly equal to the composition of theactive layer 105 at the interface with the active layer 105 and to thecomposition of the base layer 103 at the interface with the base layer103. As a consequence, the interface barrier between the active layer105 and the base layer 103 is reduced greatly compared with thesemiconductor light-emitting device according to the conventionalembodiment shown in FIG. 17. Even with a relatively low reverse voltage,therefore, the holes reaching the interface between the active layer 105and the graded composition layer 104 swiftly move to the collector layer103 so that the concentration of holes in the region of the active layer105 closer to the base layer 103 is reduced significantly. As a result,the quantity of holes accumulated in the whole active layer 105 is alsoreduced, which achieves a significant reduction in the amount ofresidual light emitted from the semiconductor light-emitting deviceduring the extinction period.

[0074] Even during the light-emitting period, the graded compositionlayer 104 provided between the active layer 105 and the base layer 103also reduces an energy barrier (spike) at the lower end of theconduction band between the active layer 105 and the base layer 103,though it is not depicted. This also improves the efficiency of electroninjection in the active layer 105 during the light-emitting period.

[0075] Thus, the first embodiment achieves a high extinction ratio witha reverse bias voltage lower than in the semiconductor light-emittingdevice according to the conventional embodiment. This enables high-speedswitching between the light-emitting operation and the light-extinctoperation.

[0076] In the semiconductor light-emitting device according to the firstembodiment, the depletion layer between the base and the collector isalso formed even during the light-emitting period, which is differentfrom the conventional light-emitting diode device. This reduces theelectrostatic capacitance between the base layer 103 and the collectorlayer 102 suitably for operation by high-speed modulation.

[0077] Because of the n-type base layer 103, majority carriers injectedfrom the base layer 103 are electrons which are higher in mobility thanholes so that resistance of charge injected in a direction parallel tothe substrate surface is reduced. On the other hand, minority carriersinjected from the p-type emitter layer 106 into the n-type base layer103 are holes which are smaller in diffusion length than electrons sothat the diffusion of the minority carriers in a direction parallel tothe substrate surface is suppressed.

[0078] Embodiment 2

[0079] A second embodiment of the present invention will be describedwith reference to the drawings.

[0080]FIG. 3 shows a cross-sectional structure of a semiconductorlight-emitting device in a triode configuration according to the secondembodiment, which is an npn-type semiconductor light-emitting devicecomposed of an InGaN/GaN-based compound semiconductor.

[0081] As shown in FIG. 3, the semiconductor light-emitting deviceaccording to the second embodiment has: an emitter layer 202 composed ofn-type gallium nitride (GaN); an active layer 203 composed of indiumgallium nitride (InGaN); a graded composition layer 204; a base layer205 composed of p-type GaN with a thickness of about 400 nm; and acollector layer 206 composed of n-type GaN, which are formedsuccessively on an insulating substrate 201 composed of sapphire(Al₂O₃).

[0082] In the second embodiment also, the graded composition layer 204provided between the active layer 203 and the base layer 205 has a filmthickness of about 5 nm to about 100 nm and a composition which isnearly equal to the composition of the active layer 203 at the interfacewith the active layer 203 and to the composition of the base layer 205at the interface with the base layer 205. Since light is emitted fromthe region of the graded composition layer 204 closer to the activelayer 203, it is also possible to regard the region as a part of theactive layer 203.

[0083] The upper surface of the emitter layer 202 is exposed and ann-type emitter electrode 209 is formed on the exposed region in spacedapart relation to the respective side surfaces of the active layer 203,the graded composition layer 204, and the like. The upper surface of thebase layer 205 is exposed and a p-type base electrode 210 is formed onthe exposed region in spaced apart relation to the side surface of thecollector layer 206.

[0084] An n-type collector layer 207 composed of n-typehigh-concentration GaAs is formed on a part of the collector layer 206.An n-type collector electrode 208 is formed on the n-type contact layer207.

[0085] In the semiconductor light-emitting device according to thesecond embodiment, a high-resistance region 202 a is formed by ionimplantation in the region of the emitter layer 202 which is opposed tothe p-type base electrode 210 and not opposed to the collector layer206. By thus providing the high-resistance region 202 a in the region ofthe n-type emitter layer 202 which is not opposed to the n-typecollector layer 206, carrier injection from the region of the n-typeemitter layer 202 which is not opposed to the n-type collector layer 206into the active layer 203 is suppressed. This prevents the deteriorationof the extinction ratio due to light emission caused by carriers(electrons) remaining as a result of insufficient carrier extractionfrom the collector layer 206 during an extinction period.

[0086] Since the insulating substrate 201 is transparent with respect tothe wavelength of emitted light, the majority of the emitted light canbe extracted to the outside from the surface of the insulating substrate201 opposite to the emitter layer 202.

[0087] Because of the npn-type configuration of the semiconductorlight-emitting device according to the second embodiment, holes aresupplied from the base layer 205 and electrons are supplied from theemitter layer 202 during the light-emitting period.

[0088] Conversely, the electrons are extracted from the active layer 203to the collector layer 206 during the extinction period so that therecombination of the electrons and the holes is halted. However, thegraded composition layer 204 provided between the active layer 203 andthe base layer 205 to have a gradually varying composition which isnearly equal to the composition of the active layer 203 at the interfacewith the active layer 203 and to the composition of the base layer 205at the interface with the base layer 205 allows swift carrier extractionat a low voltage so that high-speed modulation at a high extinctionratio is implemented at a low voltage, similarly to the firstembodiment.

[0089] Embodiment 3

[0090] A third embodiment of the present invention will be describedwith reference to the drawings.

[0091]FIG. 4 shows a cross-sectional structure of a semiconductorlight-emitting device in a triode configuration according to the thirdembodiment, which is a pnp-type semiconductor light-emitting devicecomposed of an AlGaAs-based compound semiconductor.

[0092] As shown in FIG. 4, the semiconductor light-emitting deviceaccording to the third embodiment has: a collector layer 302 composed ofp-type aluminium gallium arsenide (Al_(0.4)Ga_(0.6)As); a gradedcomposition layer 303 with a film thickness of about 70 nm;. a baselayer 304 composed of n-type GaAs with a film thickness of about 300 nm;and an emitter layer 305 composed of p-type Al_(0.3)Ga_(0.7)As, whichare formed successively on a substrate 301 composed of p-type GaAs.

[0093] The third embodiment features the graded composition layer 303provided between the collector layer 302 and the base layer 304 to havea composition which is nearly equal to the composition of the collectorlayer 302 at the interface with the collector layer 302 and to thecomposition of the base layer 304 at the interface with the base layer304. The film thickness of the graded composition layer 303 is adjustedappropriately to about 5 nm to about 100 nm.

[0094] The forbidden band as an electron energy band in the base layer304 composed of n-type GaAs is smaller in width than the forbidden bandin each of the collector layer 302 and the emitter layer 305 eachcomposed of p-type AlGaAs.

[0095] A p-type collector layer 307 is formed on the surface of thesubstrate 301 opposite to the collector layer 302.

[0096] The upper surface of the base layer 304 is exposed and an n-typebase electrode 308 is formed on the exposed region in spaced apartrelation to the side surface of the emitter layer 305.

[0097] A p-type contact layer 306 composed of p-type high-concentrationGaAs is formed on a part of the emitter layer 305. A p-type emitterelectrode 309 is formed on the p-type contact layer 306.

[0098] In the third embodiment, an impurity concentration in each of thecollector layer 302, the graded composition layer 303, and the baselayer 304 is adjusted to about 1×10¹⁷ cm⁻³. On the other hand, animpurity concentration in the emitter layer 305 is adjusted to about1×10¹⁸ cm⁻³, which is about ten times higher than the impurityconcentration in the collector layer 302. As a result, the quasi-Fermilevel of holes in the emitter layer 305 is closer to the upper end ofthe valence band than the quasi-Fermi level of holes in the collectorlayer 302, so that the efficiency of carrier injection from the emitterlayer 305 to the base layer 304 is improved compared with the efficiencyof carrier injection from the collector layer 302 to the base layer 304.If the impurity concentration in the emitter layer 305 is increased toabout double the impurity concentration in the collector layer 302, theefficiency of carrier injection from the emitter layer 305 can beincreased to a value higher than the efficiency of carrier injectionfrom the collector layer 302. The impurity concentration in the emitterlayer 305 is effective if at least the portion thereof opposed tocollector layer 302 has an impurity concentration which is double theimpurity concentration in the other portion thereof or higher.

[0099] On the other hand, p-type Al_(0.3)Ga_(0.7)As is used for theemitter layer 305 and p-type Al_(0.4)Ga_(0.6)As containing aluminium ina larger proportion than in the emitter layer 305 is used for thecollector layer 302. As a consequence, the sum of electronic affinityand forbidden band width in the electron energy bands in the collectorlayer 302 is increased by about 50 meV. In other words, the energy valueat the upper end of the valence band in the collector layer 302 becomessmaller than the energy value at the upper end of the valence band inthe emitter layer 305 by about 50 meV. If the energy value at the upperend of the valence band in the collector layer 302 is lower by about 10meV than in the emitter layer 305, a flow of holes into the collectorlayer 302 during a light-emitting period can be suppressed. In addition,a leakage current from the emitter layer 305 to the collector layer 302can be suppressed and reverse carrier injection from the collector layer302 to the base layer 304 can also be suppressed.

[0100] A description will be given herein below to the light-emittingoperation and light-extinct operation of the semiconductorlight-emitting device thus constituted.

[0101] The description will be given first to the operation during thelight-emitting period.

[0102]FIG. 5 shows the structure of electron energy bands in the baselayer 304 and its vicinity during the light-emitting period. In FIG. 5,reference numerals associated with energy levels correspond to thesemiconductor layers shown in FIG. 4. In the semiconductorlight-emitting device according to the third embodiment, an independentactive layer is not provided between the base layer 304 and the emitterlayer 305, which is different from the semiconductor light-emittingdevices according to the first and second embodiments. Since theforbidden band width is smaller in the base layer 304 than in each ofthe collector layer and the emitter layer 305, as described above,electrons as majority carriers and holes supplied from the emitter layer305 can be confined to the base layer 304 with simultaneous applicationof forward bias voltages between the base layer 304 and the emitterlayer 305 and between the base layer 304 and the collector layer 302.The recombination of the electrons and the holes in the base layer 304causes light emission. In short, the base layer 304 according to thethird embodiment has the function of transporting and supplyingelectrons and the function as an active layer in combination.

[0103] During the light-emitting period, the voltage between the baseand the collector is adjusted to a value lower by about 0.1 V than thevoltage between the base and the emitter. This allows the supply ofholes only from the emitter layer 305 and suppresses hole injection fromthe collector layer 302 to the base layer 304.

[0104] As a result, light emission does not occur in the region of thebase layer 304 from which the emitter layer 305 has been removed butoccurs in the region of the base layer 304 opposed to the emitter layer305.

[0105] Since the energy value at the upper end of the valence band inthe collector layer 302 is smaller by 10 meV or more than in the emitterlayer 305, a leakage current formed from the holes injected from theemitter layer 305 to the base layer 304 and flowing to the collectorlayer 302 can be suppressed. In addition, hole injection from thecollector layer 302 to the base layer 304 is suppressed.

[0106] The description will be given next to the operation during anextinction period.

[0107]FIG. 6 shows the structure of electron energy bands in the baselayer 304 and its vicinity during the extinction period.

[0108] During the extinction period, a forward bias voltage is appliedbetween the base layer 304 and the emitter layer 305, while the baselayer 304 and the collector layer 302 are set at equal potentials.Consequently, a depletion layer expands between the base layer 304 andthe collector layer 302 and the base layer 304 cannot confine holes atthe interface with the graded composition layer 303 any more. As aresult, the holes accumulated in the base layer 304 are released to thecollector layer 302 so that the concentration of holes in the base layer304 is reduced and the amount of light emitted from the device isreduced. The hole releasing operation is performed at a high speed sinceit is not dependent on the carrier recombination speed.

[0109] Since the semiconductor light-emitting device according to thethird embodiment has the graded composition layer 303 provided betweenthe base layer 304 and the collector layer 302 to have a graduallyvarying composition which is nearly equal to the composition of the baselayer 304 at the interface with the base layer 304 and to thecomposition of the collector layer 302 at the interface with thecollector layer 302, the interface barrier between the base layer 304and the collector layer 302 during the extinction period is reducedsignificantly compared with a device which does not have the gradedcomposition layer 303. As a result, the holes reaching the interfacebetween the base layer 304 and the graded composition layer 303 swiftlymove to the collector layer 302 so that light emission from the deviceduring the extinction period is further suppressed.

[0110] Thus, the third embodiment achieves a high extinction ratio witha low reverse bias voltage and thereby enables high-speed switchingbetween the light-emitting operation and the light-extinct operation.

[0111] In addition, the base and the collector are set at equalpotentials during the extinction period so that it is unnecessary toapply a reverse voltage. This provides a driving circuit with a simplerstructure and a easier driving method.

[0112] Embodiment 4

[0113] A fourth embodiment of the present invention will be describedwith reference to the drawings.

[0114]FIG. 7 shows a cross-sectional structure of a semiconductorlight-emitting device in a triode configuration according to the fourthembodiment, which is an npn-type semiconductor light-emitting devicecomposed of an AlGaAs-based compound semiconductor.

[0115] As shown in FIG. 7, the semiconductor light-emitting deviceaccording to the fourth embodiment has: a collector layer 402 composedof n-type Al_(0.4)Ga_(0.6)As; a graded composition layer 403 with a filmthickness of about 20 nm; a base layer 404 composed of p-type GaAs witha film thickness of about 300 nm; and an emitter layer 405 composed ofn-type Al_(0.3)Ga_(0.7)As, which are formed successively on a substrate401 composed of n-type GaAs.

[0116] The fourth embodiment features the graded composition layer 403provided between the collector layer 402 and the base layer 404 to havea composition which is nearly equal to the composition of the collectorlayer 402 at the interface with the collector layer 402 and to thecomposition of the base layer 404 at the interface with the base layer404. The film thickness of the graded composition layer 403 is adjustedappropriately to about 5 nm to about 100 nm.

[0117] The forbidden band as an electron energy band in the base layer404 composed of p-type GaAs is smaller in width than the forbidden bandin each of the collector layer 402 and the emitter layer 405 eachcomposed of n-type AlGaAs.

[0118] An n-type collector layer 407 is formed on the surface of thesubstrate 401 opposite to the collector layer 402.

[0119] The upper surface of the base layer 404 is exposed and a p-typebase electrode 408 is formed on the exposed region in spaced apartrelation to the side surface of the emitter layer 405.

[0120] An n-type contact layer 406 composed of n-type high-concentrationGaAs is formed on a part of the emitter layer 405. An n-type emitterelectrode 409 is formed on the n-type contact layer 406.

[0121] In the semiconductor light-emitting device according to thefourth embodiment, a high-resistance region 402 a is formed by ionimplantation in the region of the collector layer 402 which is opposedto the p-type base electrode 408 and is not opposed to the emitter layer405.

[0122] The impurity concentration in each of the graded compositionlayer 403 and the base layer 404 is adjusted to about 1×10¹⁷ cm⁻³, whilethe impurity concentration in the collector layer 402 is adjusted toabout 5×10¹⁷ cm⁻³. On the other hand, the impurity concentration in theemitter layer 405 is adjusted to about 1×10¹⁸ cm⁻³, which is about twiceas high as the impurity concentration in the collector layer 402. Thisimproves the efficiency of carrier injection from the emitter layer 405to the base layer 404.

[0123] The impurity concentration in the emitter layer 405 is effectiveif at least the portion thereof opposed to the collector layer 402 hasan impurity concentration which is double the impurity concentration inthe other portion thereof or higher.

[0124] In the fourth embodiment, n-type Al_(0.3)Ga_(0.7)As is used forthe emitter layer 405 and n-type Al_(0.4)Ga_(0.6)As containing aluminiumin a larger proportion than in the emitter layer 405 is used for thecollector layer 402. As a consequence, electronic affinity in theelectron energy bands in the collector layer 402 becomes smaller byabout 10 meV. In other words, the energy value at the lower end of theconduction band in the collector layer 402 becomes larger than theenergy value at the lower end of the conduction band in the emitterlayer 405 by about 10 meV. This suppresses a flow of electrons into thecollector layer 402 during a light-emitting period. In addition, aleakage current from the emitter layer 405 to the collector layer 402can be suppressed and reverse electron injection from the collectorlayer 402 to the base layer 404 can also be suppressed.

[0125] Thus, in the semiconductor light-emitting device of the fourthembodiment which is the npn-type triode device, the forbidden band widthin the base layer 404 has been adjusted to be smaller than the forbiddenband width in each of the collector layer 402 and the emitter layer 405so that the base layer 404 has a light-emitting function for generatingrecombination light, instead of providing an independent active layer.

[0126] Since the semiconductor light-emitting device according to thefourth embodiment has the conductivity type opposite to that of thesemiconductor light-emitting device according to the third embodiment,electrons are supplied from the emitter layer 405 to the base layer 404during a light-emitting period, while electrons are extracted from thebase layer 404 to the collector layer 402 during an extinction period.Since the graded composition layer 403 is provided between the baselayer 404 and the collector layer 402, the impurity concentration hasbeen adjusted to be higher in the emitter layer 405 than in thecollector layer 402 and the lower lend of the conduction band has beenadjusted to be lower in the collector layer 402 than in the emitterlayer 405, similarly to the third-embodiment, high-speed operation isperformed.

[0127] Since it is unnecessary to apply a reverse bias voltage betweenthe base and the collector during the extinction period, similarly tothe third embodiment, a simpler driving circuit can be usedappropriately.

[0128] In addition, the fourth embodiment has provided thehigh-resistance region 402 a in the region of the collector layer 402which is not opposed to the emitter layer 405 so that carrier injectionin the reverse direction from the collector layer 402 toward the baselayer 404 can be suppressed. Even if equal potentials are applied to thecollector layer 402 and the emitter layer 405, therefore, unrequiredlight emission does not occur in the peripheral portion of the baselayer 404. This provides an easier method for driving the semiconductorlight-emitting device and improves the efficiency of light emission.

[0129] Embodiment 5

[0130] A fifth embodiment of the present invention will be describedwith reference to the drawings.

[0131]FIG. 8 shows a cross-sectional structure of a semiconductorlight-emitting device in a triode configuration according to the fifthembodiment, which is a pnp-type semiconductor light-emitting devicecomposed of an AlGaAs/GaAs/GaInP-based compound semiconductor.

[0132] As shown in FIG. 8, the semiconductor light-emitting deviceaccording to the fifth embodiment has: a collector layer 502 composed ofp-type GaInP, an undoped semiconductor layer 503 consisting of acollector-side undoped layer 503 a composed of intrinsic GaInP and abase-side undoped layer 503 b composed of intrinsic GaAs and having atotal thickness of about 120 nm, a base layer 504 composed of n-typeGaAs with a thickness of about 300 nm, and an emitter layer 505 composedof p-type Al_(0.3)Ga_(0.7)As and having an upper portion patterned intoa ridge-shaped configuration.

[0133] The fifth embodiment features the undoped semiconductor layer 503provided between the collector layer 502 and the base layer 504 to havean impurity concentration of 5×10¹⁶ cm⁻³ or less.

[0134] Moreover, the width of the forbidden band as an electron energyband is smaller in the base layer 504 composed of n-type GaAs than ineach of the collector layer 502 composed of p-type GaInP and the emitterlayer 505 composed of p-type AlGaAs.

[0135] A p-type collector electrode 507 is formed on the surface of thesubstrate 501 opposed to the collector layer 502.

[0136] The upper surface of the base layer 504 is exposed and an n-typebase electrode 508 is formed on the exposed region in spaced apartrelation to the side surface of the emitter layer 505.

[0137] A p-type contact layer 506 composed of p-type high-concentrationGaAs is formed on the ridge-shaped region of the emitter layer 505. Acurrent constricting layer 510 composed of a silicon dioxide (SiO₂) isburied above the emitter layer 505 and sidewise of the ridge-shapedregion to have an upper surface nearly flush with the upper surface ofthe current constricting layer 510. A p-type emitter electrode 509 isformed on the current constricting layer 510 to come in contact with thep-type contact layer 506.

[0138] In the third embodiment, the impurity concentration in the baselayer 504 has been adjusted to about 1×10¹⁷ cm⁻³, while the impurityconcentration in each of the collector layer 502 and the emitter layer505 has been adjusted to 1×10¹⁸ cm⁻³.

[0139] On the other hand, p-type Al_(0.3)Ga_(0.7)As is used for theemitter layer 505 and p-type GaInP having a composition whichsubstantially lattice-matches with the substrate 501 composed of GaAs isused for the collector layer 502. As a consequence, the sum ofelectronic affinity and forbidden band width is larger in the electronenergy bands of the collector layer 502 than in the emitter layer 505 byabout 50 meV or more. In other words, the energy value at the upper endof the valence band in the collector layer 502 becomes smaller than theenergy value at the upper end of the valence band in the emitter layer505 by about 50 meV or more. If the energy value at the upper end of thevalence band in the collector layer 502 is lower by about 10 meV than inthe emitter layer 505, a flow of holes into the collector layer 502during a light-emitting period can be suppressed.

[0140] By thus using AlGaAs in the emitter layer 505, GaInP in thecollector layer 502, and GaAs in the base layer 504, the resistivity ofthe collector layer 502 can be reduced to a low value, while increasinga band offset due to the heterojunction interface between the individualsemiconductor layers.

[0141] Since the semiconductor light-emitting device according to thefifth embodiment has the upper surface covered with the p-type emitterelectrode 509 and the current constricting layer 510, light emitted fromthe base layer 504 is released not in the front-to-rear direction of thesubstrate 501 but from a cleaved end surface of the light-emittingdevice.

[0142] Since the fifth embodiment features the undoped semiconductorlayer 503 having an impurity concentration of 5×10¹⁶ cm⁻³ or less andprovided between the collector layer 502 and the base layer 504, aleakage current from the emitter layer 505 to the collector layer 502can be suppressed even if the respective impurity concentrations in theemitter layer 505 and the collector layer 502 are set to substantiallyequal values. Moreover, hole injection in the reverse direction from thecollector layer 502 to the base layer 504 can also be suppressed.

[0143] Since the undoped semiconductor layer 503 reduces the amount ofelectrostatic capacitance between the base and the collector during thelight-emitting period, high-speed driving can be performed easily.

[0144] Since the undoped semiconductor layer 503 sharpens a potentialgradient in the interface barrier (spike) between the base and thecollector during an extinction period, as shown in the band structure ofFIG. 9A, holes are less likely to be accumulated in the interfacebarrier so that the value of the extinction ratio is increased. FIG. 9Bis for comparison and shows a band structure during the extinctionperiod when the undoped semiconductor layer 503 is not provided betweenthe base layer 504 and the collector layer 502. As shown in FIG. 9B,when the undoped semiconductor layer 503 is not provided between thebase layer 504 and the collector layer 502, an interface barrier due toa band offset occurs at the interface between the base layer 504 and thecollector layer 502.

[0145] Since the fifth embodiment need not apply a reverse bias voltagebetween the base and the collector during the extinction period,similarly to the third embodiment, a simpler driving circuit can be usedappropriately.

[0146] Although the fifth embodiment has provided the opposing surfacesof the collector layer 502 and the base layer 504 with the respectiveundoped layers such that the collector-side undoped layer 503 a and thebase-side undoped layer 503 b compose the undoped semiconductor layer503, it is also possible to use only one of the collector-side undopedlayer 503 a and the base-side undoped layer 503 b as an undoped layer.

[0147] By controlling the impurity distribution in the base layer 504 orthe collector layer 502, the impurity concentration in the interfacewith the base layer 504 or the collector layer 502 can be suppressedeasily.

[0148] It is also possible to provide an undoped graded compositionlayer between the collector-side undoped layer 503 a and the base-sideundoped layer 503 b composing the undoped semiconductor layer 503. Thearrangement reduces the interface barrier during the extinction period.In this case, it is also possible to use only a graded composition layerto compose the undoped semiconductor layer 503 instead of using thecollector-side undoped layer 503 a and the base-side undoped layer 503b.

[0149] Although the fifth embodiment has used GaAs for the base layer504, if the forbidden band width in the base layer 504 is increased byusing Al_(x)Ga_(1-x)As (where 0<×≦0.3) containing aluminium, thewavelength of emitted light can be reduced.

[0150] Although the fifth embodiment has used AlGaAs for the emitterlayer 502, if GaInP is used similarly to the collector layer 502, theeffect of confining carriers to the base layer 504 can be enhanced.Conversely, if AlGaAs is used for the collector layer 502, the undopedsemiconductor layer 503 or the graded composition layer can be formedbetween the base layer 504 and the collector layer 502 in an easierfabrication process.

[0151] Embodiment 6

[0152] A sixth embodiment of the present invention will be describedwith reference to the drawings.

[0153]FIG. 10 shows a cross-sectional structure of a semiconductorlight-emitting device in a triode configuration according to the sixthembodiment, which is a pnp-type semiconductor light-emitting devicecomposed of a GaAs/AlGaAs-based compound semiconductor.

[0154] As shown in FIG. 10, the semiconductor light-emitting deviceaccording to the sixth embodiment has: a collector layer 602 composed ofp-type Al_(0.3)Ga_(0.7)As; an n-type lightly doped base layer 603; abase layer 604 composed of n-type GaAs with a film thickness of about300 nm; and an emitter layer 605 composed of p-type Al_(0.3)Ga_(0.7)As,which are formed successively on a substrate 601 composed of p-typeGaAs.

[0155] The width of the forbidden band as an electron energy band issmaller in the base layer 604 composed of n-type GaAs than in each ofthe collector layer 602 and the emitter layer 605 each composed ofp-type AlGaAs.

[0156] The sixth embodiment features the lightly doped base layer 603provided between the collector layer 602 and the base layer 604 andconsisting of a collector-side lightly doped layer 603 a composed ofn-type Al_(0.3)Ga_(0.7)As with a thickness of about 35 nm, a gradedcomposition layer 603 b with a thickness of about 25 nm, and a base-sidelightly doped layer 603 c composed of n-type GaAs with a thickness ofabout 10 nm, which are formed successively on the collector layer 602.The impurity concentration in each of the collector-side lightly dopedlayer 603 a, the graded composition layer 603 b, and the base-sidelightly doped layer 603 c has been adjusted to 1×10¹⁷ cm⁻³.

[0157] The graded composition layer 603 b has a composition which isnearly equal to the composition of the base-side lightly doped layer 603c at the interface with the base-side lightly doped layer 603 c and tothe composition of the collector-side lightly doped layer 603 a at theinterface with the collector-side lightly doped layer 603 a.

[0158] A p-type collector electrode 607 is formed on the surface of thesubstrate 601 opposite to the collector layer 602.

[0159] The upper surface of the base layer 604 is exposed and an n-typebase electrode 608 is formed on the exposed region in spaced apartrelation to the side surface of the emitter layer 605.

[0160] A p-type contact layer 606 composed of p-type high-concentrationGaAs is formed on a part of the emitter layer 605. A p-type emitterelectrode 609 is formed on the p-type contact layer 606.

[0161] In the sixth embodiment, the impurity concentration in the baselayer 604 is adjusted to about 1×10¹⁸ cm⁻³ and the impurityconcentration in the collector layer 605 is adjusted to about 1×10¹⁷cm⁻³. On the other hand, the impurity concentration in the emitter layer605 is adjusted to about 1×10¹⁸ cm⁻³, which is ten times higher than theimpurity concentration in the collector layer 602. As a result, theefficiency of carrier injection from the emitter layer 605 to the baselayer 604 is improved compared with the efficiency of carrier injectionfrom the collector layer 602 to the base layer 604. The impurityconcentration in the emitter layer 605 is effective in improving theefficiency of injection from the emitter layer 605 if it is increased toa value about double the impurity concentration in the collector layer602. The impurity concentration in the emitter layer 605 is effective ifat least the portion thereof opposed to the collector layer 602 has animpurity concentration which is double the impurity concentration in theother portion thereof or higher.

[0162] Preferably, the sum of electronic affinity and forbidden bandwidth is larger in the electron energy bands of the collector-sidelightly doped layer 603 a than in the base layer 604 by about 20 meV ormore. In other words, the energy value at the upper end of the valenceband in the collector-side lightly doped layer 603 a is preferablysmaller than the energy value at the upper end of the valence band inthe base layer 604 by about 20 meV or more. In the arrangement, even ifa forward bias voltage is applied between the collector and the baseduring a light-emitting period, a barrier against carriers moving fromthe collector layer 602 to the base layer 604 occurs.

[0163] If the conductivity type of the semiconductor light-emittingdevice is inverted so that the collector-side lightly doped layer 603 ahas an n-type conductivity, the energy value at the lower end of theconduction band is preferably larger by about 20 meV or more than in thebase layer 604.

[0164] A description will be given herein below to the light-emittingoperation and light-extinct operation of the semiconductorlight-emitting device thus constituted.

[0165] The description will be given first to the light-emittingoperation.

[0166]FIG. 11 shows the structure of electron energy bands in the baselayer 604 and its vicinity during the light-emitting period. In FIG. 11,reference numerals associated with energy levels correspond to thesemiconductor layers shown in FIG. 10.

[0167] During the light-emitting operation, forward bias voltages atequal potentials are applied between the base layer 604 and the emitterlayer 605 and between the base layer 604 and the collector layer 602.

[0168] In the sixth embodiment, the energy value at the upper end of thevalence band is equal in each of the emitter layer 605 and the collectorlayer 602, as shown in FIG. 11. However, the n-type lightly doped baselayer 603 is provided between the n-type base layer 604 and the p-typecollector layer 602 to form the pn junction with the collector layer602, which causes an energy barrier 600 against holes between the baselayer 604 and the collector layer 602. The energy barrier 600 preventshole injection in the reverse direction from the collector layer 602 tothe base layer 604 even if the emitter layer 605 and the collector layer602 are set at precisely equal values. This suppresses light emissionfrom the portion of the base layer 604 underlying the n-type baseelectrode 608 and from the exposed portion of the base layer 604.

[0169] Since the emitter layer 605 and the collector layer 602 can beset at equal values, the device can be driven by an easier method and aleakage current from the emitter layer 605 to the collector layer 602does not occur.

[0170] Since carrier injection in the reverse direction from thecollector layer 602 to the base layer 604 can be prevented, it is nomore necessary to provide the collector layer 402 with thehigh-resistance region 402 a as in the fourth embodiment so that thedevice is fabricated in a reduced number of process steps.

[0171] Even if the emitter layer 605 and the collector layer 602 are atdifferent potentials during the light-emitting period, the energybarrier 600 caused by the lightly doped base layer 603 suppresses aleakage current between the emitter layer 605 and the collector layer602.

[0172] Since a depletion layer is formed at the interface between thelightly doped base layer 603 and the collector layer 602 during thelight-emitting period, the amount of electrostatic capacitance betweenthe base layer 604 and the collector layer 602 is reduced so that theresponse of the device when driven at a high speed is improved comparedwith that of the device which does not have the lightly doped base layer603.

[0173] The description will be given to the light-extinct operation ofthe semiconductor light-emitting device according to the sixthembodiment.

[0174]FIG. 12 shows the structure of electron energy bands during anextinction period. In FIG. 12, reference numerals associated with energylevels correspond to the semiconductor layers shown in FIG. 10.

[0175] During the extinction period, a forward bias voltage is appliedbetween the emitter layer 605 and the base layer 604, while thecollector layer 602 and the base layer 604 are set at equal potentials.This increases the width of the depletion layer compared with the casewhere a forward bias voltage is applied between the collector layer 602and the base layer 604 and therefore the lightly doped base layer 603and the collector layer 602 cannot confine the holes any more. As aresult, the holes are extracted from the base layer 604 to the collectorlayer 602 so that a current flows in large quantity between the emitterand the collector, while the concentration of the holes in the baselayer 604 is reduced and the amount of light emitted from the base layer604 is reduced.

[0176] Since the sixth embodiment has provided the graded compositionlayer 603 b between the collector-side lightly doped layer 603 a and thebase-side lightly doped layer 603 c, the interface barrier between thecollector-side lightly doped layer 603 a and the base-side lightly dopedlayer 603 c is weakened so that the effect of extracting carriers duringthe extinction period is particularly enhanced. This allows thesemiconductor light-emitting device according to the sixth embodiment toperform high-speed light-emitting and light-extinct operations.

[0177] Since it is unnecessary to apply a reverse bias voltage betweenthe base and the collector during the extinction period and reverse holeinjection from the collector layer 602 to the base layer 604 issuppressed, not only the fabrication of the device but also the drivingmethod for the device are facilitated.

[0178] As a variation of the sixth embodiment, a structure which doesnot have the graded composition layer 603 b between the collector-sidelightly doped layer 603 a and the base-side lightly doped layer 603 cwill be described herein below. Since the arrangement obviates thenecessity to form the graded composition layer 603 b having acomposition which should be varied gradually during the growth thereof,the device can be fabricated in a reduced number of process steps.

[0179]FIGS. 13 and 14 show the band structure in a semiconductorlight-emitting device according to the present variation, of which FIG.13 shows the light-emitting period and FIG. 14 shows the extinctionperiod.

[0180] As shown in FIG. 14, an interface barrier (spike) resulting fromdiscontinuities in the energy bands occurs between the base layer 604and the collector layer 602 during the extinction period. However, sincethe spike is positioned in the center portion of the high-electric-fielddepletion layer, the influence of the spike is suppressed. As a result,the holes accumulated in the base layer 604 can be extracted reliably tothe collector layer 602 so that light emission from the device duringthe extinction period is suppressed.

[0181] In the sixth embodiment and the variation thereof, the impurityconcentration in the lightly doped base layer 603 is preferably half theimpurity concentration in the base layer 604 or less.

[0182] More preferably, the lightly doped base layer 603 has an impurityconcentration of 1×10¹⁶ cm⁻³ to 5×10¹⁷ cm⁻³ and a film thickness of 30nm to 400 nm. In the arrangement, if the emitter layer 605 and thecollector layer 602 are set at equal potentials, the lightly doped baselayer 603 functions as an energy barrier against carriers. If the baselayer 604 and the collector layer 602 are set at equal potentials, theenergy barrier can be removed so that the semiconductor light-emittingdevice according to the sixth embodiment is driven easily and reliably.

[0183] The impurity concentration in each of the collector-side lightlydoped layer 603 a, the graded composition layer 603 b, and the base-sidelightly doped layer 603 c of the lightly doped base layer 603 iscontrolled as follows. If the collector-side lightly doped layer 603 ais taken as an example, a film is formed while it is doped with p-typeimpurity ions equal to those used to dope the collector layer 602. Then,the portion of the film to be formed with the collector-side lightlydoped layer 603 a is doped with n-type impurity ions such that thecollector-side lightly doped layer 603 a has a low n-type impurityconcentration.

[0184] Although each of the first to sixth embodiment has used thelight-emitting diode device using spontaneous light emission as anexample of the semiconductor light-emitting device, the presentinvention is also applicable to an edge-emitting or surface-emittingsemiconductor laser device or the like utilizing induced light emission.

[0185] As a semiconductor material composing the semiconductorlight-emitting device, a Group III-V compound semiconductor such asGaAs, AlAs, InAs, GaP, AlP, InP, GaN, AlN, or InN may be used.Alternatively, a Group II-VI compound semiconductor such as ZnSe, CdSe,MgSe, ZnS, CdS, ZnTe, CdTe, ZnO, CdO, or MgO may be used. It is alsopossible to use a mixed crystal material of compound semiconductors suchas AlGaAs, GaInP, AlGaInP, InGaAsP, AlGaN, InGaN, ZnCdSe, or MgZnO.

[0186] Of the foregoing compound semiconductors, if a conductive one isused as the material of the substrate of the semiconductorlight-emitting device, an electrode can be formed on the surface of thesubstrate opposite to the surface formed with the device so that thedevice is fabricated in a reduced number of process steps.

[0187] If a semi-insulating substrate composed of a semi-insulating oneof the foregoing compound semiconductors or an insulating substratecomposed of sapphire, a silicon dioxide, or the like is used, theelectrostatic capacitance of the light-emitting device is reduced sothat the RF characteristic thereof is improved. If a plurality ofdevices are formed, they are insulated from each other, which allowseasy integration of the devices.

[0188] Embodiment 7

[0189] A seventh embodiment of the present invention will be describedwith reference to the drawings.

[0190]FIG. 15 shows the structure of functional blocks in an apparatusfor driving a semiconductor light-emitting device according to theseventh embodiment.

[0191] As shown in FIG. 15, the apparatus for driving the semiconductorlight-emitting device according to the seventh embodiment is ofpnp-type, which comprises: a semiconductor light-emitting device 701 ina triode configuration composed of an emitter, a base, and a collector;a constant-current generating circuit 702 as constant-current controlmeans for receiving a first power-source voltage V_(CC1) and supplying aspecified driving current I_(E) to the emitter of the semiconductorlight-emitting device 701; and a light-emission control circuit 703 aslight-emission control means 702 for receiving a control signal and asecond power-source voltage V_(CC2), controlling a potential at thecollector of the semiconductor light-emitting device 701, and therebycontrolling a state of light emitted from the semiconductorlight-emitting device 701.

[0192] The base of the semiconductor light-emitting device 701 isconnected to a first ground terminal 704 as specified-potential applyingmeans.

[0193] The constant-current generating circuit 702 supplies the drivingcurrent I_(E) to the emitter of the semiconductor light-emitting device701 such that a forward bias voltage is applied between the emitter andthe base.

[0194] The light-emission control circuit 703 switches the collector ofthe semiconductor light-emitting device 701 between a high-potentialstate or high-resistance state and a low-potential state in response tothe control signal inputted from the outside.

[0195] The semiconductor light-emitting device 701 is brought into alight-emitting state when, if a forward bias voltage applied between thecollector and the base is assumed to be a positive potential, thecollector potential is higher than the base potential, i.e., thecollector and the base are in a forward bias state or set at equalpotentials, similarly to the semiconductor light-emitting deviceaccording to the first embodiment. Conversely, the semiconductorlight-emitting device 701 is brought into a non-light-emitting statewhen the collector potential is sufficiently low, i.e., the collectorand the base are in a reverse bias state.

[0196] A description will be given herein below to a specific example ofthe driving method implemented by the apparatus for driving thesemiconductor light-emitting device according to the seventh embodimentduring light-emitting and extinction periods

[0197] The description will be given first to a light-emittingoperation.

[0198] When the light-emission control circuit 703 is operated with thecontrol signal from the outside and brought into a high-resistance statewhen viewed from the collector of the semiconductor light-emittingdevice 701, the potential at the collector of the semiconductorlight-emitting device 701 is set at an intermediate potential betweenthe base and emitter potentials, whereby the semiconductorlight-emitting device 701 is brought into the light-emitting state. Inaccordance with another method, e.g., the collector of the semiconductorlight-emitting device 701 is grounded to a second ground terminal 705 sothat the collector and base are set at equal potentials, whereby thesemiconductor light-emitting device 701 is brought into thelight-emitting state.

[0199] The description will be given next to a light-extinct operation.

[0200] When the second power-source voltage V_(CC2) is applied to thecollector of the semiconductor light-emitting device 701 by operatingthe light-emission control circuit 703 with the control signal from theoutside, the collector and the base are brought into the reverse biasstate. If the second power-source voltage V_(CC2) is sufficiently highin the reverse bias state, the majority of charge injected from theemitter is extracted to the collector so that the carrier density in theactive layer is reduced. Once the effect of confining the carriers tothe active layer lowers and the extraction of the carriers to thecollector is initiated, a current injected from the emitter tends toincrease. In the seventh embodiment, however, a constant amount ofcurrent is supplied to the emitter under the control of theconstant-current generating circuit 702.

[0201] Accordingly, the forward bias voltage between the emitter and thebase lowers and the quasi-Fermi level at the opposing surface of theemitter composing the interface between the emitter and the activeregion lowers. This reduces the carrier density not only at theinterface between the active region and the base but also at theinterface between the active region and the emitter, so that the carrierdensity in the active region is reduced significantly and light emissionfrom the semiconductor light-emitting device 701 is further suppressed.

[0202] Thus, the seventh embodiment easily and reliably increases theextinction ratio of the semiconductor light-emitting device 701 in atriode configuration.

[0203] It is also possible to use, e.g., an npn-type bipolar transistoras an example of the light-emission control circuit 703 according to theseventh embodiment. By connecting the collector terminal of the bipolartransistor to the collector of the semiconductor light-emitting device701 and connecting the emitter terminal to the second power-sourcevoltage V_(CC2) such that the control signal is inputted to the baseterminal, the light-emission control circuit 703 can easily beimplemented.

[0204] A pnp-type bipolar transistor can also be used for thelight-emission control circuit 703. In this case, the emitter terminalof the bipolar transistor is connected to the collector of thesemiconductor light-emitting device 701 and the collector terminal isconnected to the second power-source voltage V_(CC2) such that thecontrol signal is inputted to the base terminal.

[0205] The configuration of the light-emission control circuit 703 isnot limited to the npn-type or pnp-type bipolar transistor. Thelight-emission control circuit 703 may also be composed of a multi-stagetransistor. By using a field-effect transistor (FET), ahigh-electron-mobility transistor (HEMT), or the like, a more stable andhigher-speed driving operation can be performed. Therefore, the circuitconfiguration is not limited provided that the function of thelight-emission control circuit 703 is implemented.

[0206] Embodiment 8

[0207] An eighth embodiment of the present invention will be describedwith reference to the drawings.

[0208]FIG. 16 shows the structure of functional blocks in an apparatusfor driving a semiconductor light-emitting device according to theeighth embodiment.

[0209] As shown in FIG. 16, the apparatus for driving the semiconductorlight-emitting device according to the eighth embodiment is of pnp-type,which comprises: a semiconductor light-emitting device 801 in a triodeconfiguration composed of an emitter, a base, and a collector; aconstant-current generating circuit 802 as constant-current controlmeans for receiving a first power-source voltage V_(CC1) and supplying aspecified driving current I_(E) to the emitter of the semiconductorlight-emitting device 801; and a light-emission control circuit 803 aslight-emission control means for receiving a control signal, controllinga potential at the emitter or collector of the semiconductorlight-emitting device 801, and thereby controlling a state of lightemitted from the semiconductor light-emitting device 801.

[0210] The base of the semiconductor light-emitting device 801 isconnected to a first ground terminal 804 as specified-potential applyingmeans.

[0211] The constant-current generating circuit 802 supplies a drivingcurrent I_(E) to the emitter of the semiconductor light-emitting device801 such that a reverse bias voltage is applied between the emitter andthe base.

[0212] The light-emission control circuit 803 switches the collector ofthe semiconductor light-emitting device 801 between a high-potentialstate or high-resistance state and a low-potential state in response tothe control signal inputted from the outside.

[0213] The semiconductor light-emitting device 801 is brought into alight-emitting state when, if a forward bias voltage applied between thecollector and the base is assumed to be a positive potential, thecollector potential is higher than the base potential, i.e., thecollector and the base are in a forward bias state, similarly to thesemiconductor light-emitting device according to the third embodiment.Conversely, the semiconductor light-emitting device 801 is brought intoa non-light-emitting state when the collector and the base are set atnearly equal potentials.

[0214] A description will be given herein below to a specific example ofthe driving method implemented by the apparatus for driving thesemiconductor light-emitting device according to the eighth embodimentduring light-emitting and extinction periods.

[0215] The description will be given first to a light-emittingoperation.

[0216] When the light-emission control circuit 803 is operated with thecontrol signal from the outside and brought into a high-resistance statewhen viewed from the collector of the semiconductor light-emittingdevice 801, the potential at the collector of the semiconductorlight-emitting device 801 is set at a potential nearly equal to theemitter potential and brought into a forward bias state relative to thebase so that emission occurs. In this case, the light-emission controlcircuit 803 need not be connected to the emitter of the semiconductorlight-emitting device 801. In accordance with another method, thecollector of the semiconductor light-emitting device 801 is connecteddirectly to the emitter so that the collector and the base are broughtinto a forward bias state and light emission occurs.

[0217] The description will be given next to a light-extinct operation.

[0218] When the collector of the semiconductor light-emitting device 801is connected to a second ground terminal 805 and a ground potential isapplied by operating the light-emission control circuit 803 with thecontrol signal from the outside, carriers cannot be confined between thecollector and the base any more. As a result, the majority of chargeinjected from the emitter is extracted to the collector so that thecarrier density in the base is reduced. Since a constant amount ofcurrent is supplied to the emitter under the control of theconstant-current generating circuit 802, similarly to the seventhembodiment, the forward bias voltage between the emitter and the base isreduced, the carrier density in the base is reduced significantly, andlight emission from the device is further suppressed.

[0219] Thus, the eighth embodiment easily and reliably increases theextinction ratio of the semiconductor light-emitting device 801 in atriode configuration.

[0220] It is also possible to use, e.g., an npn-type bipolar transistoras an example of the light-emission control circuit 803 according to theeighth embodiment. By connecting the collector terminal of the bipolartransistor to the collector of the semiconductor light-emitting device801 and connecting the emitter terminal to the second ground terminal805 such that the control signal is inputted to the base terminal, thelight-emission control circuit 803 can easily be implemented. In thiscase, a control voltage of, e.g., 0 V (ground potential) is used as thecontrol signal during the light-emitting operation and a positivecontrol voltage of, e.g., 0.8 V or more is used as the control signalduring the extinction period. As a result, the collector terminal of thebipolar transistor is brought into a high-resistance state during thelight-emitting period and into a low-potential state (low-resistancestate) close to a ground state during the extinction period. This allowscontrol of the light-emitting and light-extinct operations according tothe present embodiment.

[0221] A pnp-type bipolar transistor can also be used for thelight-emission control circuit 803. Specifically, the emitter terminalof the bipolar transistor is connected to the collector of thesemiconductor light-emitting device 801 and the collector terminal isconnected to the second ground terminal 805 such that the control signalis inputted to the base terminal. In this case, a control voltage higherthan the emitter potential during the light-emitting period of thesemiconductor light-emitting device 801, e.g., is used as the controlsignal during the light-emitting period and a control voltage of, e.g.,0 V (ground potential) is used as the control signal during theextinction period. As a result, the emitter terminal of the bipolartransistor is brought into a high-resistance state during thelight-emitting period and into a low-potential state (low-resistancestate) at about 0.7 V close to the ground state during the extinctionoperation. This allows control of the light-emitting and light-extinctoperations according to the present embodiment.

[0222] The configuration of the light-emission control circuit 803 isnot limited to the npn-type or pnp-type bipolar transistor. Thelight-emission control circuit 803 may also be composed of a multi-stagetransistor. By using a field-effect transistor (FET), ahigh-electron-mobility transistor (HEMT), or the like, a more stable andhigher-speed driving operation can be performed. Therefore, the circuitconfiguration is not limited provided that the function of thelight-emission control circuit 803 is implemented.

[0223] Although each of the seventh and eighth embodiment has describedthe example in which the pnp-type triode semiconductor light-emittingdevice is driven with the positive power source, the present inventionis also applicable to the driving of the npn-type triode semiconductorlight-emitting device. It is also possible to provide a substantiallyequal circuit configuration by simply reversing the polarity of thepower source.

[0224] Although each of the seventh and eight embodiment has connectedthe base of the semiconductor light-emitting device in the triodeconfiguration directly to the first ground terminal, it is also possibleto improve controllability over the light-emitting and light-extinctoperations by inserting a resistor, a diode device, or the like betweenthe base and the first ground terminal and thereby increasing the basepotential such that it is higher than the ground potential.

[0225] In each of the seventh and eight embodiments, it is also possibleto provide a differentiation circuit composed of a resistor and anelectrostatic capacitance or the like for the control signal inputtedfrom the outside and thereby improve the rising characteristic at theinitiation of the light-emitting operation and the fallingcharacteristic at the halt of the light-emitting operation (during theextinction period).

What is claimed is:
 1. A semiconductor light-emitting device comprising:first and second semiconductor layers each of a first conductivity type;a third semiconductor layer of a second conductivity type providedbetween the first and second semiconductor layers; an active layerprovided between the second and third semiconductor layers, the activelayer emitting light with charge injected therein from the second andthird semiconductor layers; and a graded composition layer providedbetween the active layer and the third semiconductor layer to have avarying composition which is nearly equal to a composition of the activelayer at an interface with the active layer and to a composition of thethird semiconductor layer at an interface with the third semiconductorlayer.
 2. A semiconductor light-emitting device comprising: first andsecond semiconductor layers each of a first conductivity type; a thirdsemiconductor layer of a second conductivity type provided between thefirst and second semiconductor layers, the third semiconductor layerhaving a forbidden band as an electron energy band which is smaller inwidth than a forbidden band in each of the first and secondsemiconductor layers; and a graded composition layer provided betweenthe first and third semiconductor layers to have a varying compositionwhich is nearly equal to a composition of the first semiconductor layerat an interface with the first semiconductor layer and to a compositionof the third semiconductor layer at an interface with the thirdsemiconductor layer, the third semiconductor layer emitting light withcharge injected therein from the second and third semiconductor layers.3. The semiconductor light-emitting device of claim 2, wherein animpurity concentration in the second semiconductor layer is higher atleast in a region thereof opposed to the first semiconductor layer thanin the first semiconductor layer.
 4. A semiconductor light-emittingdevice comprising: first and second semiconductor layers each of ap-type conductivity; and a third semiconductor layer of an n-typeconductivity provided between the first and second semiconductor layers,the third semiconductor layer having a forbidden band as an electronenergy band which is smaller in width than a forbidden band in each ofthe first and second semiconductor layers, the third semiconductor layeremitting light with charge injected therein from the second and thirdsemiconductor layers, an energy value at an upper end of a valence bandas an electron energy band being lower in the first semiconductor layerthan in the second semiconductor layer.
 5. The semiconductorlight-emitting device of claim 4, wherein an impurity concentration inthe second semiconductor layer is higher at least in a region thereofopposed to the first semiconductor layer than in the first semiconductorlayer.
 6. A semiconductor light-emitting device comprising: first andsecond semiconductor layers each of an n-type conductivity; and a thirdsemiconductor layer of a p-type conductivity provided between the firstand second semiconductor layers, the third semiconductor layer having aforbidden band as an electron energy band which is smaller in width thana forbidden band in each of the first and second semiconductor layers,the third semiconductor layer emitting light with charge injectedtherein from the second and third semiconductor layers, an energy valueat a lower end of a conduction band as an electron energy band beinghigher in the first semiconductor layer than in the second semiconductorlayer.
 7. The semiconductor light-emitting device of claim 6, wherein animpurity concentration in the second semiconductor layer is higher atleast in a region thereof opposed to the first semiconductor layer thanin the first semiconductor layer.
 8. A semiconductor light-emittingdevice comprising: first and second semiconductor layers each of a firstconductivity type; a third semiconductor layer of a second conductivitytype provided between the first and second semiconductor layers, thethird semiconductor layer having a forbidden band as an electron energyband which is smaller in width than a forbidden band in each of thefirst and second semiconductor layers; and a lightly doped semiconductorlayer provided between the first and third semiconductor layers, thelightly doped semiconductor layer having an impurity concentration whichis lower than an impurity concentration in each of the first and thirdsemiconductor layers, the third semiconductor layer emitting light withcharge injected therein from the second and third semiconductor layers.9. The semiconductor light-emitting device of claim 8, wherein thelightly doped semiconductor layer is an undoped layer undoped with animpurity.
 10. The semiconductor light-emitting device of claim 8,wherein the lightly doped semiconductor layer has the secondconductivity type.
 11. An apparatus for driving a semiconductorlight-emitting device comprising first and second semiconductor layerseach of a first conductivity type and a third semiconductor layer of asecond conductivity type provided between the first and secondsemiconductor layers, the apparatus comprising: constant-current controlmeans; light-emission control means for controlling a state of lightemitted from the semiconductor light-emitting device; andspecified-potential applying means for applying a specified potential tothe third semiconductor layer of the semiconductor light-emittingdevice, the constant-current control means supplying a specified drivingcurrent to the second semiconductor layer of the semiconductorlight-emitting device, the light-emission control means adjusting anamount of light emitted from the semiconductor light-emitting device byapplying different voltages to the first semiconductor layer or bybringing the first semiconductor layer into different states ofimpedance.